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Hi doodlebug52.
With the weather you are camping in, there will be additional work no matter that you do.

Leaving the valve open will surely allow for a build up of solids in the tank. The solids will need to be thoroughly cleaned out on a regular basis.

If closing the valve allows the contents to freeze, there is a danger of cracking the tank. In addition you'll need to clean out the frozen contents to continue to use the toilet.

If it was me, I'd leave the valve closed. I'd minimize the threat of the tank contents freezing by any means locally available to you. The light you mentioned is a good start. Bales of hay (if allowed) protecting the underside of the RV is something else you could try. There are other methods and after market devices you can try.

Cold weather camping does bring additional challenges. Good luck.

__________________
Gary
2005 Newmar KSDP 3910 + GMC ENVOY XUV 37K lbs Moving Down The Road
The Avatar Is Many Times Around The USANobody Knows Your Coach Like Somebody Who Owns One Just Like Yours

If you remember the old "Get Smart" TV series, they always used the "Cone of Silence". If you leave a black tank valve open you form a cone of waste in the tank just below the toilet.

Imagine a cross section of your toilet drain and the tank below. As the solid from the toilet falls with the liquid, the solids just sits below the toilet (paper and poop) and the liquid (flush water and pee) run off and down the waste drain (if it's left open). Now, before the next use, that solid waste that stuck, starts to harden. After several of these cycles, you start to form a cone or reverse volcano if you will. Eventually that cone reaches the top of that tank and the bottom of the toilet drain, when that happens, the liquids nor solids have anywhere to go but up.

This is bad in both winter and summer, in the winter the cone tends to freeze and in the summer the cone hardens faster. If you ever form the "Cone of Silence", it's kind of like a silent passing of gas; silent but deadly. Leave the black tank valve closed at all times until full and ready to dump, then let her rip!

A solution in the winter; after dumping of the black tank and closing the drain valve, add some RV storage antifreeze down the toilet. This will keep the contents from freezing until dumped again.

Be sure to close the gray valve before dumping the black, if you get the waste from the black flowing into the gray at the same time it's going down the sewer hose, the gray will start to smell as well and the oder can come back through the traps in the sink and shower.

When we had a TT permanently parked in a CG years ago for weekend and vacation use, we left our black tank valve open. However, we (as did all the regulars) had plumbed the valve outlet to the sewer drain with 3" rigid PVC pipe. Also, I installed an internal tank flushing nozzle and did a thorough tank flush every Sunday before we left for home (or after any 3 or 4 day stay).

While that worked for us for 5 years, I am not sure I would try that again. I believe the best solution is to leave the valve closed until the tank is at least 3/4 full, then empty and flush thoroughly.

The black water drain valve should be left colsed until the tank is two thirds or more full. This helps avoid the solids building up right under the toilet and assists flushing everything out. If the tank is not 2/3rds full when it is time to break camp, add water through the toilet. After the black water tank is drained and flushed close the valve and add enough water to cover the bottom of the tank and then add the tank chemical.

Very helpful advice! I'm retiring to a TT in January up here on Vancouver Island in BC where freezing doesn't happen often or at least for very long. Any other advice is most welcome in living year round in my TT

Also, most new rigs have a back flush connection that may or may not do a complete job of "cleaning." After I have back flushed thoroughly I'll ask DW to fill the bowl twice and flush after each filling. Watching the clear elbow I have just before the "slinky" I will notice more "brown liquid" being forced out. Nothing is perfect.

Gee! This is a "crappy" subject.

__________________
Wayne MSGT USMC (Ret) & Earlene (CinCHouse) RVM14
2015 Winnebago Tour 42QD
It is what it is, and then it is what you make of it.

I agree with everyone, leave the black valve closed until the tank is nearly full, then empty it and flush to be safe. If you are the gambling type and feel lucky, leaving it open is the other option.

__________________2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX retired;Good Sam Life member,FMCA." My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy

Having camped in up to -8 with an open bottom camper, and -15 with an enclosed "polar package" camper, I can say that the potential risk of building the dreaded cone really outweighs the frozen valve issue. For one, if you are in freezing temps, every time you dump, put in two gallons of RV antifreeze into your empty tank.
My usual trick was to then wait until the warmest day of the month (I live by myself, so I really only dump once a month) and then open my valve. Sometimes, just a little bit would come out, meaning I had a nice big pile of brown ice in my tank (this only happened when it was -15 out). I solved this issue with my tank backflush sprayers. It took a while, but it broke down the frozen stuff and moved it out.
As for the valve, they make many RV specific gagdets for 'cold weather' but I find that heat tape for residential pipes has worked best for me. I wrap my hose it in and anything else I'm worried about freezing. It's not powerful, but that little heater plus a little insulation go a long way.

Well, best of luck, let me know if you have any other questions, because I've probably been there and done that as far as winter camping goes!

Thanks everyone for the great replies. Our valve has been closed since the first reply. lol.

A totally different question: why do BOTH of our sky lights/ceiling vents--one above our bed and other in bathroom--drip everyone morning?? The seals look tight and we had redone the seal on the bedroom vent. Yet...both drip once the sun is up every morning for, let's say, an hour. There's no snow up there, even though it's cold outside.